The conventional screwdriver is not entirely effective in design in that the head of a screw and a tip of the screwdriver engaging the screw are often deformed or even damaged when an excessive force is applied. However, certain conventional screwdrivers are provided with two contact surfaces to which the handle and the blade are fastened. When the screwdriver is at work to tighten a screw, the center of the blade serves as a pivot such that the handle and the blade are capable of turning in relation to each other at the time when the applied torsional moment is greater than the pre-set torsional moment. The pre-set torsional moment is confined by the grade of a spring which urges the contact surfaces. The precision of the pre-set torsional moment is often undermined by the wear of the contact surfaces and the fatigue of the spring.